Many speech coders today use a voice activity detector (VAD) to detect the presence of speech amid various types of background noise. Detection of voice activity assists communications in many ways. While there is no speech present, no information is transmitted, thus less information occupies the communication channel. Transmitting less information results in a savings in battery charge. Because many speech coders are implemented in a Digital Signal Processor (DSP), the speech coder can consume a lot of current while in full operation. If the VAD could be modified to turn off the speech coder while no speech is detected, this would save current that may be crucial to a portable communication unit which derives all of its power from a battery. The VAD may also be modified to turn off other communication unit functions that are not needed when there is no speech to transmit, resulting in additional battery savings.
A processing error associated with VADs is clipping at the tail (end) of a speech burst. Clipping of speech bursts degrades the speech quality and intelligibility and is easily noticed. Such clipping is prevalent when the final sound of a word is soft and blends in with the noise in the background. To fix this problem, a fixed time period, called a hangover time, has been used to keep the transmitter on after the VAD no longer detects speech activity. Using a fixed time period has its drawbacks. In periods of high signal-to-noise ratio, a fixed hangover time is too long, and wastes the communication channel. When the signal-to-noise ratio is low, the fixed hangover time can be too short, causing the communication to be clipped, resulting in a choppy-sounding communication.
Listening to a pattern of speech bursts separated by completely soundless gaps is difficult to listen to. Note that "silence" refers to the absence of speech or voice rather than total absence of sound. The VAD measures the background noise at the transmitter immediately after a speech burst. A background noise characteristic, such as the energy of the background noise, is measured and transmitted with the speech bursts to the receiver. The receiver uses this background noise characteristic to reproduce the background noise. This reproduced noise is used to fill in the gaps between speech bursts, and the listener hears a more pleasing sound at the speaker. If this measurement is taken over a very short interval, it will be susceptible to bad reproduction due to quick changes in the background noise.
Accordingly, a VAD which turns a speech coder on and off, incorporates a variable hangover time, and uses an improved background noise measurement convention is desired.